1. Field of the Invention
This invention relates to laser interferometric lithography and more particularly an arrangement for changing fringe spacing.
2. Description of Related Art
In a two beam interferometric lithography system, an angle between the two interfering laser beams defines fringe spacing for a specific setup. Fringe spacing, in turn, defines between lines and the size of features that can be produced from the setup.
Fringe spacing in an interferometer is represented by a known equation: EQU 2dsin.theta.=.lambda.
where .lambda. is the wavelength of the light being used, .theta. is the half angle between the two interfering laser beams, and d is the fringe spacing (pitch is the center-to-center distance between adjacent fringes).
The width of the fringes, or fringe spacing, defines the ultimate size of the photoresist feature that is obtained. The technique of interference lithography works by exposing linear fringe patterns on a photoresist layer, coated onto the desired substrate. Two sets of linear fringes are used for forming either photoresist dots or holes. Typically, the substrate is exposed to two orthogonal linear fringe patterns. As a result, criss-cross patterns form. Upon development, either photoresist dots or holes are left depending upon the tone of the photoresist.
Most applications of interferometric lithography require varying the pitch and the feature size, to some extent, to accommodate the process involved. Varying the pitch and feature size requires changing the optical arrangement. Such a change of the optical arrangement is a very difficult, labor-intensive, time-consuming procedure and therefore is counter productive. In essence, a change of fringe spacing amounts to changing the angle subtended by the incident optical paths of the two laser beams, which interfere on the surface of the sample. This change requires precise, realignment of the entire optical setup.